The process disclosed by the present invention finds its motivation in improving the Methanol-to-Gasoline (“MTG”) scheme Methanol-to-Olefin and Gasoline middle Distillate (MOGD), first described by Exxon-Mobil, Inc. The original Mobil MTG process is as follows: (1) synthesis gas is converted to methanol; (2) the methanol is converted to dimethylether (“DME”); and (3) the DME is converted to the longer hydrocarbon chains through the replacement of carbon-oxygen bonds with carbon-carbon bonds. The key part of the process is the use of a proprietary catalyst, ZSM-5, to assist in the cleavage of DME to selectively form hydrocarbons chains, with the water produced as a byproduct. As described below, there are a number of patented variations on this fundamental process, as the art has attempted to improve such characteristics as reaction yield, ease of processing, and the preferential production of higher octane rated hydrocarbons in the catalyzed reaction.
The prior art describes many alternative processes to produce gasoline and distillate from synthesis gas that do not anticipate the present invention, which discloses three reaction stages with an overall recycle loop to produce commercial quality fuel. As discussed in greater detail in U.S. patent application Ser. No. 12/942,680, there are multiple differing processes for producing commercially viable fuel from synthesis gas. Chang et al. (U.S. Pat. No. 4,076,761) discloses a two-stage process wherein synthesis gas is conveyed to a CO/CO2 converter, and then to a fuel-producing stage. Garwood et al. (U.S. Pat. No. 4,304,951) further discloses the use of ZSM-5 catalyst for hydrotreatment of the heavy fraction of hydrocarbon product in an independent, isolated step. Both of the foregoing patents reference a four-stage process, but require manual refinements outside of the closed system, resulting in undesirable process complexities and low yield of the preferred end-products.
A major issue with the Mobil MTG process and prior art has been that the proprietary catalyst used in the reaction, ZSM-5, preferentially produces an increasing percentage of durene and other excessively heavy aromatics at higher pressures. These compounds, by virtue of their high freezing points, are undesirable in commercially used fuel and gasoline. While operating the reaction in a lower pressure environment would conceivably obviate the issue, this resolution is not possible because the primary precursor to the catalyzed reaction, methanol, is equilibrium limited and preferentially produced at only higher pressures. Thus, without operating the reaction at higher pressures, there would be an insufficient supply of precursor methanol, and thus insufficient DME, for the catalyzed reaction to yield an acceptable quantity of hydrocarbon end-product. Consequently, literature contemplated an isomerization step to reduce durene levels, but never demonstrated its implementation. J. Topp-Jorgenson, “Topsoe Integrated Gasoline Synthesis—the TIGAS Process,” D. M. Bibby, C. D. Chang, R. W. Howe, & S. Yurchak (eds.), Methane Conversion, 1988, Elsevier Science Publishers, B.V., Amsterdam, 293-305. This article was the first to discuss not only a possible isomerization step for reduction of durene in the end-product, but also to combine the first three stages of the Mobil MTG process into a single synthesis gas recycle loop, obviating the need for separation of intermediates.
Other artisans have attempted to improve the Mobil MTG process by integrating the first three stages of the process into a single step. F. Simard, U. A. Sedran, J. Sepulveda, N. S. Figoli, H. I. de Lasa, Applied Catalysis A: General 125 (1995):81-95. Integration of the steps required the use of a combined synthesis gas/methanol catalyst along with a methanol/gasoline catalyst. The authors utilized a combined ZnO—Cr2O3/ZSM-5 catalyst for the process. This process, however, preferentially produced extremely high levels of CO2, rather than the sought-after long-chain hydrocarbons. The cumulative reaction is 2nCO+nH2→(CH2)n+nCO2. Javier Erena et al., Chem. Engineering Sci. 55 (2000) 1845-1855.
The foregoing examples serve to demonstrate the complexity of commercially viable applications of the Mobil MTG process. Even Mobil's attempts to commercialize the process, while marginally successful, begs for enhancements to reduce complexities and inefficiencies in the conversion process. Yurchak in D. M. Bibby, C. D. Chang, R. W. Howe, & S. Yurchak (eds.), Methane Conversion, 1988, Elsevier Science Publishers, B.V., Amsterdam, 251-272. In the process outlined by the authors, the process is broken into discrete components. First, synthesis gas is converted to a methanol/water mixture at an independent site. The mixture is sent to a holding site, while the exothermic output of the reaction is recycled and utilized as a heat sink to drive conversion of synthesis gas. The methanol/water mixture is removed from the first reaction by cooling and separating it out of the synthesis gas. This mixture is then sent to a two-stage reactor system that converts the methanol to DME, and a recycle-loop MTG reactor that converts the methanol/DME mixture to both preferred and non-preferred fuel products. In particular, heavy gasoline, primarily in the form of durene, a 1,2,4,5-tetramethyl benzene molecule that has a high freezing point (79.3° C.), must be removed to make a viable gasoline product. This removal is achieved through a discrete hydrotreatment process, requiring elevated pressure, a presulfided catalyst and recycling of hydrogen gas through the system. The hydrotreatment process required three separation steps involving distillation, pressure modulation, and separation of intermediates. As a consequence, the hydrotreatment required for efficient production of preferred end-products was costly, complex, and time-consuming. A summary of the reaction implemented in New Zealand by Mobil using this process is provided below.
TABLE 1(a)Prior Art MTG Reaction SequenceTypical ReactorTypical ReactorTemperature, C.Pressure, AtmPrincipal ReactionsFeedCatalystsNote (1)Note (2)CO + H2   CH3OH + H2OCO, H2Reduced230-29050-100CuO/ZnO/Al2O3CH3OH   (CH3)2O + H2OCH3OH, H2Oγ-Al2O3310-32018-22 pressureCH3OH   (CH2)n + n H2OCH3OH, (CH3)2O,ZSM-5350-36618-22n/2(CH3)2O   (CH2)n + n/2H2OH2ODurene  iso-Durene(CH2)n, H2Sulfided  22-27030-40Ni—W onSiO2/Al2O3/faujasite
It is known in the art that durene, durene isomers, and other highly methyl substituted aromatics are produced in the Mobil MTG process. The hydrocarbon synthesis catalyst, ZSM-5, tends to produce unexpectedly large quantities of durene, in particular. In the commercialization of this process, these unfavorable hydrocarbon products are conventionally hydrotreated to decrease substitution. These catalyzed hydrotreatment reactions are typically performed as equilibrium reactions because it has been found that reaching the equilibrium mixture is sufficient to produce commercially viable gasoline. The hydrotreatment reaction utilized by Mobil, however, results in certain competitive dealkylation of highly substituted benzenes, decreasing the yield of usable, preferred end-products.
U.S. Pat. No. 8,686,206 to Fang et al., incorporated herein by reference, discloses a four stage process for fuel synthesis (MTGH process), with a recycle loop. In that application, the Fang et al. disclose a four-stage sequential catalytic reaction with intermediate heat exchange, but no inter-stage separation. While that invention, operates at an elevated pressure, it discloses four discrete stages. The hydrocarbon synthesis takes place in the third stage of the reaction, and the hydrotreatment occurs subsequently in the fourth stage. Conditions in the third and fourth stage differ, such that each stage contains a different catalyst and each has a different preferential environmental temperature. Simply merging the third and fourth stage reactions of Fang et al. would result in a failure to produce the desired end-products in commercially viable quantities.
Therefore, there remains a need for an improved process to produce fuel from synthesis gas, whereby the fuel contains low amounts of durene and highly substituted benzenes for better viscometric properties in cold temperature performance.